The present invention relates generally to imaging systems and method and more particularly to a system and method for millimeter and sub-millimeter wave imaging.
In the field of detection and-or imaging of concealed objects, millimeter and sub millimeter wave (hereinafter referred to as xe2x80x9cs-mmwxe2x80x9d) radiation has very interesting properties when compared to optical, microwave and radio signals. This radiation has superior propagation in poor weather conditions (e.g., rain, fog, smoke, chemical gases, dust, etc.). Moreover, it can penetrate tissue, plastic materials, different textiles, different soils and other opaque media. The wavelength is short enough to provide sufficient resolution in the images and to enable the construction of optic-like detection and imaging systems, yielding compact and lightweight systems.
S-mmw radiation has enormous potential in safety applications: weapons, drugs, contraband and chemical explosion detection in secured areas such as airports, courthouses, banks, schools, and others. Furthermore, the issue of tracking millions of undiscovered mines in former battlefields is challenging. In all of these situations, a failure to detect harmful, concealed objects can have catastrophic consequences.
Conventional inspection techniques for detecting weapons and contraband carried by persons at the entries and exits of secured areas make use of simple systems sensitive to induction changes in the observed area Most of these simple techniques are restricted to a binary (yes/no) detection of the presence of metallic items only, without any details, features or positional information about the object. These systems cannot be used for imperceptible, efficient and real-time contraband detection. The widespread availability of plastic and ceramic weapons, combined with the desire to detect explosives, plastic mines and other contraband make these conventional detection systems less useful. In order to reliably detect and visualize this new class of weapons, mines as well as drugs or explosives, radically new techniques are required.
Millimeter wave imaging of objects scattering, reflecting, or emitting the radiation in this frequency range is one of the promising possibilities. This promise is due, in part, to the fact that millimeter waves penetrate clothes, without exhibiting (known) harmful effects on the human body. The reflection and attenuation characteristics for (sub) millimeter wave radiation of the human body are substantially different from the same characteristics of ceramic and plastic weapons and narcotics. This enables the imaging of objects made of these materials and concealed on a person. Metallic objects also reflect millimeter wave radiation differently than the human body.
Systems have been developed for the real-time visualization of (covered) objects by means of quasi-optical s-mmw imaging systems. Passive as well as active systems have been and are further under development. The basic building blocks, which can be identified in existing systems, are schematically presented in FIGS. 1a, 1b and 1c. 
Referring first to the passive system of FIG. 1c, the multi-element detector array 5 receives radiation 8 by means of focusing lens 4. Radiation 8 is emitted by the person carrying covered object 3 and also includes the ambient radiation 9 reflected and scattered by the covered object. For indoor applications, however, the temperature contrast between the body 8 and ambient radiation 9 is quite small such the difference between metal, plastics explosives and human skin will be substantially difficult to differentiate. Hence, there is a need for an efficient low cost active illumination system.
Active systems developed up to now are based on the illumination of the object by means of radiation whose coherence level was lowered in an inefficient way. Referring now to FIG. 1a, radiation source 2a generates quasi-coherent s-mmw radiation that is directed towards a rotating diffuser 2b. Rotating diffuser 2b has a random conductive surface. The rotating diffuser is used to destroy the spatial coherence of the incident radiation beam and to redirect it towards an observed object 3.
In another implementation of an s-mmw illumination device shown in FIG. 1b, it has been proposed to use a spatially distributed array 1 of point sources in place of the combination of point source 2a and rotating diffuser 2b. The point sources of the array are sources of quasi-monochromatic radiation with slightly different central frequencies of the emitted radiation (the frequency distribution does not have to be larger than the normal manufacturing variations to achieve the interrelated results). The array of sources 2a/2b or 1 both are able to produce an illumination of the object 3 by radiation with decreased spatial coherence.
Referring to both FIGS. 1a and 1b, the object image is projected on a multi-element detector array 5 by means of a focusing element 4. An array of electrical signals is generated by the detector array 5 and processed (e.g., mixed, amplified, filtered) by electronic means 6 so that the object image can be visualized by the displaying means 7.
While a prior art active imaging system may in principle lead to good visual quality images, a high performance implementation is not sufficiently practical due to the inefficient nature of the coherence destruction mechanism. The fundamental reason is that the degree of spatial coherence of the illuminating radiation at the plane of the object strongly depends on the ratio between the size of the array of the spatially distributed non-coherent sources and the distance between said array and the object. Due to this dependence, the array size should be sufficiently large when imaging objects are at practical distances. Consequently only for the largest and, hence most expensive arrays, may the best imaging results be obtained.
A second drawback of prior art s-mmw imaging systems is the use of rotating diffusers in order to destroy coherence. The very bulky rotating diffusers need to rotate continuously and sufficiently fast The effectiveness of such reduction of the spatial coherence level is not high. Hence more acceptable and effective approaches for quality imaging of concealed objects are desired.
Another drawback of both these systems is the very limited possibility of using multi-frequency radiation for object illumination. To develop a multi-element array source in which every element will be able to emit radiation within sufficiently broad-band spectral range is practically non-achievable task. The random conductive surfaces of a diffuser cannot reflect the radiation in the same manner within a wide spectral range. Even if a known rotating diffuser could be developed to operate within such a wide spectral range, it would still have all of the problems of currently commercially available rotating diffusers.
Said in another way, prior art active s-mmw imaging systems are limited in effectively implementing multi-frequency illumination of the object. In the case of the multi-element source it is practically unfeasible to construct a multi-element array source, whereby every element would be able to emit radiation within a sufficiently broad spectral range. In the case of a rotating diffuser, the random conductive surface cannot reflect the radiation in the same way for the whole spectral range. Even if some modification of such mechanical diffuser will be developed the problem of rotating such bulky diffuser still holds.
In prior art active and passive systems, only one image (or eventually two in the case images for different polarization states are taken) is available. This can be stated as xe2x80x9cone- or two-parameterxe2x80x9d partial millimeter wave imaging. However, an image could be decomposed in much more partial images, whereby each partial image represents an image for a set of combinations of physical parameters of the illuminating radiation. These physical parameters are e.g. the carrier frequency of the illumination, the polarization state and the angle of incidence. Such an extended set of partial images for different combinations of the physical parameters allows much better analysis of the objects and clutter in the obtained images because one has access to each of these components. Having access to the partial components of the images allows to optimize the weighted combinations of the components (e.g. neglecting the bad components). In conventional imaging techniques such decomposition and advanced image analyzing techniques are not available.
Because X-ray is ionizing radiation, infrared radiation is non-penetrative through clothes and microwave radiation exhibits wavelengths which are too large to carry a needed volume of information concerning contraband objects, the proposed s-mmw imaging technique and apparatus are suitable realizations of remotely controlled and real time contraband detection.
The present invention provides a number of novel features that can be used advantageously in imaging systems. For example, in security applications, use of the present invention allows better imaging for concealed objects such as weapons and/or drugs. This system could be useful, for example, in airports where prior art passive systems are not as effective.
In one aspect, the present invention provides an apparatus for imaging. At least one source of composite radiation illuminates a field of view. The radiation includes a set of multiple phase-independent partials that are independently controllable and exhibit distinct physical features. A quasi-optical element is disposed between the field of view and a multi-element receiver. The multi-element receiver is disposed to receive image radiation from the quasi-optical element. Particular ones of the receiver elements transform the image radiation into a set of electrical signals that include information relating to features of the partials.
In one embodiment, an s-mmw imaging system includes a non-rotating diffuser that destroy the spatial coherence of radiation incident on the diffuser and directs the radiation towards a field of view. At least one radiation source is disposed to illuminate the diffuser. In the preferred embodiment, the radiation source(s) generates radiation having a wavelength between about 0.1 mm and about 10 mm. A quasi-optical element is disposed between the field of view and a multi-element receiver. The quasi-optical element directs radiation from the field of view toward an imaging plane. A multi-element receiver is disposed in the imaging plane such that particular ones of the receiver elements transform radiation into a set of electrical signals.
In another aspect, the present invention provides a method of illuminating a field of view. Radiation, preferably with a wavelength greater than about 0.1 mm is generated. The radiation includes multiple phase-independent partial components that exhibit distinguishable physical features. The radiation is encoded to label different ones of the multiple partial components. The radiation is then directed toward a field of view and focused on an imaging plane. The radiation can then be detected from the imaging plane and converted into electrical signals. Information relating to features of the multiple partial components can then be extracted from the electrical signals.
In yet another embodiment, the present invention provides a millimeter wave system that includes a source of radiation The radiation includes a set of independently controllable radiation components. Each radiation component includes a doublet that includes two spectral lines. Each radiation component is also labeled by a given frequency shift between the two spectral lines. The system also includes a receiver with an array of receiver elements disposed to receive the radiation emitted by the source. The receiver transforms the received radiation into an array of electrical signals. A processing system can be used to decode the array of electrical signals based on the labels of the radiation components.
As will be discussed throughout the text, aspects of the present invention provide a number of advantages over prior art systems. For example, by utilizing the techniques of the present invention, clearer images are possible in a variety of contexts. Other aspects of the present invention can be used in other applications such as in communication systems.